1. Field of the Invention
The present invention relates to air intake systems for internal combustion engines, and, more particularly, to an air intake system which is adapted for controlling the temperature of the combustion air by means of a flow proportioning valve which admits cold raw air and preheated raw air into the air intake system at an adjustable ratio.
2. DESCRIPTION OF THE PRIOR ART
It is now common practice to equip internal combustion engines, especially those used in vehicles, with devices for controlling the temperature of the combustion air. Such a device admixes preheated raw air to the raw air of ambient temperature, in order to obtain improved fuel combustion at low ambient temperatures, and in order to decrease the emission of pollutants during engine warmup.
While a relatively high air temperature contributes to better vaporization of the fuel, and hence to a more complete combustion, it is desirable to use combustion air of a somewhat lower temperature for full-load operation, because the higher density of cold air produces a higher power output by increasing the volumetric efficiency of the engine. It is therefore generally considered optimal to have a combustion air temperature of approximately 20.degree. C.-30.degree. C. during warmup and during partial-load operation and to allow a considerably lower combustion air temperature for full-load operation, at low ambient temperatures.
The known prior art devices utilize an air flow proportioning valve in the intake duct system which admits cold raw air and preheated raw air from an exhaust stove which is associated with the engine exhaust manifold. The operation of the flow proportioning valve may be controlled by a thermostat, e.g. a wax thermostat whose expansion and contraction displaces the flow proportioning valve. Alternatively, the valve may be driven by a pneumatic actuator device which utilizes the negative pressure of the engine intake manifold as a power source. Since the level of this negative pressure fluctuates with the level of engine load--maximum at idle and minimum at full load--it is necessary to add to the pneumatic actuator various corrective features, in addition to conventional temperature sensing means which operate to relieve the negative pressure, in response to an increase in the temperature of the combustion air which enters the carburetor.
In view of the aforementioned difficulties which accompany the use of the engine manifold vacuum as the motive power for the flow proportioning valve, attempts have been made to replace it with a different source of power. One such proposal is disclosed in German Auslegeschrift (Publ. Allowed Application) No. 20 17 983, where a wax thermostat has its push rod connected to the flapper of an air flow proportioning valve, while being surrounded by a heater coil whose heat output is controlled by a temperature sensor in the air intake system and an associated signal transformer.
A major shortcoming of this known device is the slowness of response of the wax thermostat which requires a considerable amount of time to expand during a heating phase and to contract, after the electric current to the heater coil has been shut off. In fact, the wax thermostat has a tendency to continue expanding, even after shutoff of the heating current. It follows that this device produces undesirable fluctuations in the resulting air temperature. In addition, it offers no possibility for the adjustment of a lower combustion air temperature for full-load operation.